Related strontium isotopic and chemical variations in oceanic basalts

Sr⁸⁷/Sr⁸⁶ values in oceanic basalts range from 0.7012 to 0.7057 and correlate with basalt composition as measured by the ratio K₂O/(Na₂O + K₂O). The distribution of data points on this plot can be approximated by the following ranges in Sr⁸⁷/Sr⁸⁶ and K₂O/(K₂O + Na₂O) respectively: (l) ocean ridge tholeiites—0.7020 to 0.7035 (one value 0.7012), <0.10; (2) many island tholeiites and alkali basalts—0.7030 to 0.7045, 0.10 to 0.30; and (3) potassic island basaltic rocks—0.7040 to 0.7057, >0.30. If the volcanism occurring throughout much of geologic time preferentially depleted rubidium and potassium relative to strontium in the mantle, preservation of the resultant heterogeneities is necessary to explain the isotopic and chemical differences among oceanic basalts. As a corollary to this long-term depletion of rubidium and potassium of the mantle, the primitive mantle or total crust-mantle system would have an Sr⁸⁷/Sr⁸⁶ value higher than many oceanic basalts derived from zones that have undergone multistage histories. Therefore, we suggest that the potassic lavas with Sr⁸⁷/Sr⁸⁶ higher than those of ocean ridge tholeiites and many island basalts represent the least depleted or most primitive mantle sampled by young oceanic volcanism.